arXiv subjects

Detlef Lohse

Publications and source records attributed to Detlef Lohse.

At least 361 records · Page 20Linked to original sources

arXiv2012

Microdroplet impact at very high velocity

Water microdroplet impact at velocities up to 100 m/s for droplet diameters from 12 to 100 um is studied. This parameter range covers the transition from capillary-limited to viscosity-limited spreading of the impacting droplet. Splashing is absent for all measurements; the droplets always gently spread over the surface. The maximum spreading radius is compared to several existing models. The model by Pasandideh-Fard et al. agrees well with the measured data, indicating the importance of a thin boundary layer just above the surface, in which most of the viscous dissipation in the spreading droplet takes place. As explained by the initial air layer under the impacting droplet, a contact angle of 180 degrees is used as model input.

physics.flu-dyn↗

arXiv2012

Maximal air bubble entrainment at liquid drop impact

At impact of a liquid drop on a solid surface an air bubble can be entrapped. Here we show that two competing effects minimize the (relative) size of this entrained air bubble: For large drop impact velocity and large droplets the inertia of the liquid flattens the entrained bubble, whereas for small impact velocity and small droplets capillary forces minimize the entrained bubble. However, we demonstrate experimentally, theoretically, and numerically that in between there is an optimum, leading to maximal air bubble entrapment. Our results have a strong bearing on various applications in printing technology, microelectronics, immersion lithography, diagnostics, or agriculture.

physics.flu-dyn↗

arXiv2012

Logarithmic temperature profiles in turbulent Rayleigh-Bénard convection

We report results for the temperature profiles of turbulent Rayleigh-Bénard convection (RBC) in the interior of a cylindrical sample of aspect ratio $Γ\equiv D/L = 0.50$ ($D$ and $L$ are the diameter and height respectively). Results from experiment over the Rayleigh number range $4\times 10^{12} \alt Ra \alt 10^{15}$ for a Prandtl number $\Pra \simeq 0.8$ and from direct numerical simulation (DNS) at $Ra = 2 \times 10^{12}$ for $\Pra = 0.7$ are presented. We find that the temperature varies as $A*ln(z/L) + B$ where $z$ is the distance from the bottom or top plate. This is the case in the classical as well as in the ultimate state of RBC. From DNS we find that $A$ in the classical state decreases in the radial direction as the distance from the side wall increases and becomes small near the sample center.

physics.flu-dyn↗

arXiv2012

Highly focused supersonic microjets

The paper describes the production of thin, focused microjets with velocities up to 850 m/s by the rapid vaporization of a small mass of liquid in an open liquid-filled capillary. The vaporization is caused by the absorption of a low-energy laser pulse. A likely explanation of the observed phenomenon is based on the impingement of the shock wave caused by the nearly-instantaneous vaporization on the free surface of the liquid. An experimental study of the dependence of the jet velocity on several parameters is conducted, and a semi-empirical relation for its prediction is developed. The coherence of the jets, their high velocity and good reproducibility and controllability are unique features of the system described. A possible application is to the development of needle-free drug injection systems which are of great importance for global health care.

physics.flu-dyn↗

arXiv2012

Fluctuation theorems for an asymmetric rotor in a granular gas

We investigate the validity of fluctuation theorems for an asymmetric rotor experiment in a granular gas. A first state, with a Gaussian distribution of the angular velocity, is found to be well described by a first order Langevin equation. We show that fluctuation theorems are valid for the injected work and for the total entropy production. In a second state the angular velocity distribution is double-peaked due to a spontaneous symmetry breaking: A convection roll develops in the granular gas, which strongly couples to the rotor. Surprisingly, in this case similar symmetry relations hold, which lead to a good prediction for the height ratio of the two peaks.

cond-mat.stat-mech↗

arXiv2012

Surface nanobubbles: Seeing is believing

The existence of surface nanobubbles has been previously suggested using various experimental techniques, including attenuated total reflection spectroscopy, quartz crystal microbalance, neutron reflectometry, and x-ray reflectivity, but all of these techniques provide a sole number to quantify the existence of gas over (usually) hundreds of square microns. Thus `nanobubbles' are indistinguishable from a `uniform gassy layer' between surface and liquid. Atomic force microscopy, on the other hand, does show the existence of surface nanobubbles, but the highly intrusive nature of the technique means that a uniform gassy layer could break down into nanobubbles \textit{due to} the motion of the microscope's probe. Here we demonstrate \textit{optical} visualisation of surface nanobubbles, thus validating their individual existence non-intrusively.

cond-mat.soft↗

arXiv2012

Spatial distribution of heat flux and fluctuations in turbulent Rayleigh-Bénard convection

We numerically investigate the radial dependence of the velocity and temperature fluctuations and of the time-averaged heat flux $\bar{j}(r)$ in a cylindrical Rayleigh-Bénard cell with aspect ratio Γ=1 for Rayleigh numbers Ra between $2 \times 10^6$ and $2\times 10^{9}$ at a fixed Prandtl number Pr = 5.2. The numerical results reveal that the heat flux close to the sidewall is larger than in the center and that, just as the global heat transport, it has an effective power law dependence on the Rayleigh number, $\bar{j}(r)\propto Ra^{γ_j(r)}$. The scaling exponent $γ_j(r)$ decreases monotonically from 0.43 near the axis ($r \approx 0$) to 0.29 close to the side walls ($r \approx D/2$). The effective exponents near the axis and the side wall agree well with the measurements of Shang et al. (Phys.\ Rev.\ Lett.\ \textbf{100}, 244503, 2008) and the predictions of Grossmann and Lohse (Phys.\ Fluids \textbf{16}, 1070, 2004). Extrapolating our results to large Rayleigh number would imply a crossover at $Ra\approx 10^{15}$, where the heat flux near the axis would begin to dominate. In addition, we find that the local heat flux is more than twice as high at the location where warm or cold plumes go up or down, than in the plume depleted regions.

physics.flu-dyn↗

arXiv2012

Building micro-soccer-balls with evaporating colloidal fakir drops

Evaporation-driven particle self-assembly can be used to generate three-dimensional microstructures. We present a new method to create these colloidal microstructures, in which we can control the amount of particles and their packing fraction. To this end, we evaporate colloidal dispersion droplets on a special type of superhydrophobic micro-structured surface, on which the droplet re- mains in Cassie-Baxter state during the entire evaporative process. The remainders of the droplet consist of a massive spherical cluster of the microspheres, with diameters ranging from a few tens up to several hundreds of microns. We present scaling arguments to show how the final particle packing fraction of these balls depends on the dynamics of the droplet evaporation.

cond-mat.soft↗

arXiv2012

Collapse of a non-axisymmetric, impact-created air cavity in water

The axisymmetric collapse of a cylindrical air cavity in water follows a universal power law with logarithmic corrections. Nonetheless, it has been suggested that the introduction of a small azimuthal disturbance induces a long term memory effect, reflecting in oscillations which are no longer universal but remember the initial condition. In this work, we create non-axisymmetric air cavities by driving a metal disc through an initially-quiescent water surface and observe their subsequent gravity-induced collapse. The cavities are characterized by azimuthal harmonic disturbances with a single mode number $m$ and amplitude $a_m$. For small initial distortion amplitude (1 or 2% of the mean disc radius), the cavity walls oscillate linearly during collapse, with nearly constant amplitude and increasing frequency. As the amplitude is increased, higher harmonics are triggered in the oscillations and we observe more complex pinch-off modes. For small amplitude disturbances we compare our experimental results with the model for the amplitude of the oscillations by Schmidt et al. (2009) and the model for the collapse of an axisymmetric impact-created cavity previously proposed by Bergmann et al. (2009b). By combining these two models we can reconstruct the three-dimensional shape of the cavity at any time before pinch-off.

physics.flu-dyn↗

arXiv2012

Three-dimensional Lagrangian Voronoi analysis for clustering of particles and bubbles in turbulence

Three-dimensional Voronoi analysis is used to quantify the clustering of inertial particles in homogeneous isotropic turbulence using data from numerics and experiments. We study the clustering behavior at different density ratios and particle response times (i.e. Stokes numbers St). The Probability Density Functions (PDFs) of the Voronoi cell volumes of light and heavy particles show a different behavior from that of randomly distributed particles -i.e. fluid tracers-implying that clustering is present. The standard deviation of the PDF normalized by that of randomly distributed particles is used to quantify the clustering. Light particles show maximum clustering for St around 1-2. The results are consistent with previous investigations employing other approaches to quantify the clustering. We also present the joint PDFs of enstrophy and Voronoi volumes and their Lagrangian autocorrelations. The small Voronoi volumes of light particles correspond to regions of higher enstrophy than those of heavy particles, indicating that light particles cluster in higher vorticity regions. The Lagrangian temporal autocorrelation function of Voronoi volumes shows that the clustering of light particles lasts much longer than that of heavy or neutrally buoyant particles. Due to inertial effects, the Lagrangian autocorrelation time-scale of clustered light particles is even longer than that of the enstrophy of the flow itself.

physics.flu-dyn↗

arXiv2012

Thermal boundary layer profiles in turbulent Rayleigh-Bénard convection in a cylindrical sample

We numerically investigate the structures of the near-plate temperature profiles close to the bottom and top plates of turbulent Rayleigh-Bénard flow in a cylindrical sample at Rayleigh numbers Ra=10^8 to Ra=2\times10^{12} and Prandtl numbers Pr=6.4 and Pr=0.7 with the dynamical frame method [Q. Zhou and K.-Q. Xia, Phys. Rev. Lett. 104, 104301 (2010)] thus extending previous results for quasi-2-dimensional systems to 3D systems for the first time. The dynamical frame method shows that the measured temperature profiles in the spatially and temporally local frame are much closer to the temperature profile of a laminar, zero-pressure gradient boundary layer according to Pohlhausen than in the fixed reference frame. The deviation between the measured profiles in the dynamical reference frame and the laminar profiles increases with decreasing Pr, where the thermal BL is more exposed to the bulk fluctuations due to the thinner kinetic BL, and increasing Ra, where more plumes are passing the measurement location.

physics.flu-dyn↗

arXiv2012

Lagrangian statistics of light particles in turbulence

We study the Lagrangian velocity and acceleration statistics of light particles (micro-bubbles in water) in homogeneous isotropic turbulence. Micro-bubbles with a diameter of 340 microns and Stokes number from 0.02 to 0.09 are dispersed in a turbulent water tunnel operated at Taylor-Reynolds numbers (Re) ranging from 160 to 265. We reconstruct the bubble trajectories by employing three-dimensional particle tracking velocimetry (PTV). It is found that the probability density functions (PDFs) of the micro-bubble acceleration show a highly non-Gaussian behavior with flatness values in the range 23-30. The acceleration flatness values show an increasing trend with Re, consistent with previous experiments (Voth et al., JFM, 2002) and numerics (Ishihara et al., JFM, 2007). These acceleration PDFs show a higher intermittency compared to tracers (Ayyalasomayajula et al., Phys. Fluids, 2008) and heavy particles (Ayyalasomayajula et al., Phys. Rev. Lett., 2006) in wind tunnel experiments. In addition, the micro-bubble acceleration autocorrelation function decorrelates slower with increasing Re. We also compare our results with experiments in von Karman flows and point-particle direct numerical simulations with periodic boundary conditions.

physics.flu-dyn↗

arXiv2011

Angular momentum transport and turbulence in laboratory models of Keplerian flows

We present angular momentum transport (torque) measurements in two recent experimental studies of the turbulent flow between independently rotating cylinders. In addition to these studies, we reanalyze prior torque measurements to expand the range of control parameters for the experimental Taylor-Couette flows. We find that the torque may be described as a product of functions that depend only on the Reynolds number, which describes the turbulent driving intensity, and the rotation number, which characterizes the effects of global rotation. For a given Reynolds number, the global angular momentum transport for Keplerian-like flow profiles is approximately 14% of the maximum achievable transport rate. We estimate that this level of transport would produce an accretion rate of $\dot{M}/\dot{M_0} \sim 10^{-3}$ in astrophysical disks. We argue that this level of transport from hydrodynamics alone could be significant.

physics.flu-dyn↗

arXiv2011

Direct measurements of air layer profiles under impacting droplets using high-speed color interferometry

A drop impacting on a solid surface deforms before the liquid makes contact with the surface. We directly measure the time evolution of the air layer profile under the droplet using high-speed color interferometry, obtaining the air layer thickness before and during the wetting process. Based on the time evolution of the extracted profiles obtained at multiple times, we measure the velocity of air exiting from the gap between the liquid and the solid, and account for the wetting mechanism and bubble entrapment. The present work offers a tool to accurately measure the air layer profile and quantitatively study the impact dynamics at a short time scale before impact.

physics.flu-dyn↗

arXiv2011

Drop impact on superheated surfaces

At impact of a liquid droplet on a smooth surface heated above the liquid's boiling point, the droplet either immediately boils when it contacts the surfaces (``contact boiling''), or without any surface contact forms a Leidenfrost vapor layer towards the hot surface and bounces back (``gentle film boiling''), or both forms the Leidenfrost layer and ejects tiny droplets upward (``spraying film boiling''). We experimentally determine conditions under which impact behaviors in each regime can be realized. We show that the dimensionless maximum spreading $γ$ of impacting droplets on the heated surfaces in both gentle and spraying film boiling regimes shows a universal scaling with the Weber number $\We$ ($γ\sim\We^{2/5}$) -- regardless of surface temperature and of liquid properties -- which is much steeper than for the impact on non-heated (hydrophilic or hydrophobic) surfaces ($γ\sim\We^{1/4}$). We also intereferometrically measure the vapor thickness under the droplet.

physics.flu-dyn↗

arXiv2011

Ultimate Turbulent Taylor-Couette Flow

The flow structure of strongly turbulent Taylor-Couette flow with Reynolds numbers up to Re_i = 2*10^6 of the inner cylinder is experimentally examined with high-speed particle image velocimetry (PIV). The wind Reynolds numbers Re_w of the turbulent Taylor-vortex flow is found to scale as Re_w ~ Ta^(1/2), exactly as predicted for the ultimate turbulence regime, in which the boundary layers are turbulent. The dimensionless angular velocity flux has an effective scaling of Nu_ω ~ Ta^0.38, also in correspondence with turbulence in the ultimate regime. The scaling of Nu_ω is confirmed by local angular velocity flux measurements extracted from high-speed PIV measurements: though the flux shows huge fluctuations, its spatial and temporal average nicely agrees with the result from the global torque measurements.

physics.flu-dyn↗

arXiv2011

Effect of Plumes on Measuring the Large Scale Circulation in Turbulent Rayleigh-Bénard Convection

We studied the properties of the large-scale circulation (LSC) in turbulent Rayleigh-Bénard (RB) convection by using results from direct numerical simulations in which we placed a large number of numerical probes close to the sidewall. The LSC orientation is determined by either a cosine or a polynomial fit to the azimuthal temperature or azimuthal vertical velocity profile measured with the probes. We study the LSC in Γ=D/L=1/2 and Γ=1 samples, where D is the diameter and L the height. For Pr=6.4 in an aspect ratio Γ=1 sample at $Ra=1\times10^8$ and $5\times10^8$ the obtained LSC orientation is the same, irrespective of whether the data of only 8 or all 64 probes per horizontal plane are considered. In a Γ=1/2 sample with $Pr=0.7$ at $Ra=1\times10^8$ the influence of plumes on the azimuthal temperature and azimuthal vertical velocity profiles is stronger. Due to passing plumes and/or the corner flow the apparent LSC orientation obtained using a cosine fit can result in a misinterpretation of the character of the large-scale flow. We introduce the relative LSC strength, which we define as the ratio between the energy in the first Fourier mode and the energy in all modes that can be determined from the azimuthal temperature and azimuthal vertical velocity profiles, to further quantify the large-scale flow. For $Ra=1\times10^8$ we find that this relative LSC strength is significantly lower in a Γ=1/2 sample than in a Γ=1 sample, reflecting that the LSC is much more pronounced in a Γ=1 sample than in a Γ=1/2 sample. The determination of the relative LSC strength can be applied directly to available experimental data to study high Rayleigh number thermal convection and rotating RB convection.

physics.flu-dyn↗

arXiv2011

Horizontal Structures of Velocity and Temperature Boundary Layers in 2D Numerical Turbulent Rayleigh-Bénard Convection

We investigate the structures of the near-plate velocity and temperature profiles at different horizontal positions along the conducting bottom (and top) plate of a Rayleigh-Bénard convection cell, using two-dimensional (2D) numerical data obtained at the Rayleigh number Ra=10^8 and the Prandtl number Pr=4.4 of an Oberbeck-Boussinesq flow with constant material parameters. The results show that most of the time, and for both velocity and temperature, the instantaneous profiles scaled by the dynamical frame method [Q. Zhou and K.-Q. Xia, Phys. Rev. Lett. 104, 104301 (2010) agree well with the classical Prandtl-Blasius laminar boundary layer (BL) profiles. Therefore, when averaging in the dynamical reference frames, which fluctuate with the respective instantaneous kinematic and thermal BL thicknesses, the obtained mean velocity and temperature profiles are also of Prandtl-Blasius type for nearly all horizontal positions. We further show that in certain situations the traditional definitions based on the time-averaged profiles can lead to unphysical BL thicknesses, while the dynamical method also in such cases can provide a well-defined BL thickness for both the kinematic and the thermal BLs.

physics.flu-dyn↗